I fully get that. What does herd immunity do?
That when one infected person has contact with a few locals (shopping in a local store) they likely came in contact with immune individuals or even if one of them was susceptible, their contacts were likely immune and therefore the spread chain comes to a dead end. However, if the infected person comes in contact with many people, like meshulachim going from door to door, "herd immunity" is useless. Similarly, in a camp that is not limited to "in-towners", or those that have personal immunity, herd-immunity, is useless.